I did what David is suggesting.  I have similar set up installed in the front
of my Syncro plus I upgraded the stock 4" speakers as well.
I used Clarion speaker model # SEC671 ($224.00) which consist of  6" woofers
and 1" tweeters.
The 6" woofers are mounted in the lowest door cut out with some slight
modifications and the 1" tweeters are mounted overhead on the A/C vent housing
facing down toward the driver and passenger respectively.  The wires for the
tweeters are run though the van to the rear pillar and then up into the A/C
duct back to the front.  The kit comes with a capacitor for the tweeters and
mounting bracket for directional adjustments.  Very, very happy with this
arrangement.  The lows are low and the highs are high!

I also replaced the stock 4" speaker in the upper door cut out with Clarion
model #SEM471 ($75.00) 4" midrange speakers with capacitor.  The window crank
handle does drag a bit on the grills but not enough to bother me.  (The mids
are mid!)

My system is a few years old now so the speaker model numbers and prices may
not current.  I know Clarion and Boston Acoustic both make these types of
speakers as well as some others, I'm sure.  I also have other components like
a receiver/tape deck, power amps, 4-1/2 speakers in rear A/C housing,
subwoofer, and 6 disc CD player that make it a complete sound system.  I spent
more than I wanted but I had the extra Syncro drivetrain and road noise to
overcome!

Andrew